With an increase in population and with increasing industrialization of the world, protection of environment and waters is becoming ever more important. In several countries, such as Finland, Soviet Union and Canada, the wood-processing industry is the biggest polluter of the waters.
The present invention is in particular concerned with a development of the effluent treatment of the woodrooms of large paper and pulp mills so that the circulating water in the woodroom can be made to remain clean, even so clean that the woodroom can be kept in a closed water circulation. To-day, a large pulp mill, e.g. one that produces 400,000 tons of pulp per year, uses about 2.4 million cubic meters solid measure of wood per year. The woodroom uses clean water as a quantity of about 1 cubic meter per cubic meter solid measure of barked wood, i.e. 2.4 million cubic meters of clean water per year. This quantity of water, which contains about 1.5 kg of organic fibre per cubic meter, is passed to the clarification plant of the mill.
The woodroom yields annually about 3,500 tons of organic fibre in the effluent of the woodroom, i.e. about 1.8 to 2.0 percent of the total weight quantity of the wood used.
In the following, a woodroom provided with a barking drum will be described by way of example. The woodroom and its water circulation system are shown schematically, viewed from the side of the barking drum, in FIG. 1.
Having been washed by means of jets 3 of circulating water, the wood 2 to be barked passes into the barking drum 1 for barking. The barked white wood 4 comes out of the drum and is washed with a jet 5 of clean water. Underneath the drum, there is a flight conveyor 6 with perforated bottom, the circulating water 7 flowing through the perforated bottom of the said conveyor into the circulating water channel 8. The bark 9 that comes out of the slots in the drum 1 remains on the perforated bottom of the flight conveyor 6 and is carried by the flight conveyor into the shredder 10. In the drum 1, some of the bark is ground to fine powder and to thin fibres, which pass along with the circulating water 7 into the channel 8.
The bark coming from the shredder is again carried by the flight conveyor 11 up and fed into the bark dewatering press 12, from which the conveyor 13 carries the dewatered bark further for combustion. Water that contains bark dregs flows into the channel 8 from underneath the conveyor 11 through the perforated bottom, flow 14, and to a greater extent from underneath the bark press, bark water flow 15. The largest flow into the channel is the circulating-water flow 16.
The barking drum, the shredder, and the bark press, from which the circulating water comes, are mounted on the floor level A of the woodroom so that the waters coming from all of these units flow into the same channel 8, from which the water flows into the water clarification equipment of the woodroom and into the water clarification plant of the factory.
In the barking methods commonly used today--half-wet barking drum and lopped paper or pulp wood--the yield of bark is about 0.3 cubic meters loose measure of bark per cubic meter solid measure of barked wood.
Owing to the high cost of the work taking place in the forest, in several places attempts are made to bring the trees with their branches and needles or leaves to the factory for barking. This barking of complete trees increases the quantity of bark, depending on the proportion of branches on the trees. In Canada, the quantity of bark and various branch dregs obtained has been 1 cubic meter loose measure per cubic meter solid measure of barked wood. In particular, the proportion of small particles and fibres ground from twigs, leaves and needles is increased.
Thus, the barking of complete trees makes the proportion of small particles in the bark 2-fold or 3-fold as compared with the present system. This constitutes the greatest drawback of these woodrooms and of their effluent treatment.
By means of the simple method in accordance with the present invention, it is possible to reduce the quantity of these small fibres ending up in the circulating water to an essential extent.
In terms of the need for protection of water resources, the invention is valuable. The invention is also valuable for the industry, because it does not cause high costs.
The invention is characterized in the procedure disclosed in the main claim.
In the following description and in FIG. 2, an embodiment of the invention will be illustrated, without in any way confining the invention to this particular embodiment.